PW2 What's "UPDATE ERROR: 12"

I have PW2 Japanese 4GB version, and it worked so great before 5.6.1 update.

Is there a proper way to downgrade it?

I've found that system won't delete update files if I reboot device while plugged in and but there is always problem.

I've tried a lot of versions.

First I've tried wrong versions for PW1 not PW2 and there was "UPDATE ERROR: 11" (I've found it was a wrong version later)

Now I think I've tried few right ones but I still get an error "UPDATE ERROR: 12"

Could someone just translate it to me what does it mean?